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Objective: To analyze the associations of exposure to single pollutants and their mixture with metabolic dysfunction-associated steatotic liver disease (MASLD) and MASLD with significant liver fibrosis in adults in China. Methods: A total of 23 806 participants from the third resurvey of the China Kadoorie Biobank were included. An elastic-net logistic regression model was used to calculate the weighted environmental risk score (WERS). Logistic regression models were used to evaluate the associations of the exposures to single pollutant [fine particulate matter (PM_2.5), inhalable particulate matter (PM₁₀), nitrogen dioxide (NO₂), and warm-season ozone (O₃)], and their mixture (WERS) with the risk for MASLD and MASLD with significant liver fibrosis. Restricted cubic spline models were used to evaluate non-linear relationships. Logistic regression models were also used to estimate the population attributable fraction (PAF) for air pollutants and major risk factors. Results: The mean age of the study participants was (65.3±9.1) years. Among them, 13 428 had MASLD, including 2 457 with significant liver fibrosis. Exposure to PM_2.5, PM₁₀, NO₂, and warm-season O₃ was positively associated with MASLD risk, with ORs of 1.27 (95%CI: 1.05-1.53), 1.77 (95%CI: 1.36-2.31), 1.20 (95%CI: 1.08-1.33), and 1.53 (95%CI: 1.20-1.95) per 1-SD increase, respectively. PM_2.5 showed a non-linear association with MASLD risk (P for nonlinear=0.007). The association between PM_2.5 and MASLD was mild at low exposure levels but increased obviously at higher concentrations. PM₁₀ and warm-season O₃ exposures were positively associated with significant liver fibrosis in MASLD patients. Each 1-unit increase in WERS (representing simultaneous 1-SD increases in the four pollutants) was associated with a 40% higher risk for MASLD (95%CI: 18%-65%) and a 42% higher risk for significant liver fibrosis (95%CI: 1%-100%). Compared with people with low exposure to air pollutants, the PAFs for MASLD in those with higher exposures to PM_2.5, PM₁₀, and warm-season O₃ ranged from 25.4% to 66.4%. Conclusion: Exposures to PM_2.5, PM₁₀, NO₂, and warm-season O₃, as well as their mixture, were associated with increased risk for MASLD and MASLD with significant liver fibrosis in adults in China.